Since silicone rubber has excellent characteristics including heat resistance, freeze resistance, safety, electric insulation and weather resistance, it finds widespread use in a variety of fields, for example, as automotive parts such as connector seals and spark plug boots, electric/electronic parts such as copier rolls and microwave oven gaskets, and building parts such as sealants, as well as many consumer parts such as nursing nipples and diving gears. In many of these applications, silicone rubber is used as articles combined with metals and organic resins. Many methods were proposed for manufacturing cured products of addition curable silicone rubber compositions integrated with metals and organic resins. Such integrally bonded articles are typically produced by (i) coating the surface of a molded resin with a primer, applying an uncured silicone rubber composition thereto, and curing, (ii) coating an adhesive to an interface, (iii) two-color molding so as to provide mutual engagement, and (iv) curing a self-adhesive silicone rubber composition to a molded resin. However, the use of adhesives or primers adds to the number of steps, and with a certain coating technique, the surface to be bonded may be smeared. The two-color molding method can impose limitation on the shape of integrated articles and lead to insufficient interfacial adhesion. One solution is a self-adhesive type silicone rubber composition obtained by adding an adhesive to a silicone rubber composition. The self-adhesive type silicone rubber composition eliminates the priming or coating step, shortens the operating time, reduces the cost, and improves the efficiency of operation. It constitutes an effective means for manufacturing integrally molded articles with resins.
In connection with the primerless molding of addition type heat-curable silicone rubber compositions, a number of reports have been made for bonding silicone rubber to organic resins. In one exemplary attempt, self-adhesive silicone rubber compositions are cured on resins. Many proposals were made relating to specific adhesive components for use in the self-adhesive silicone rubber compositions. In JP-8 H02-034311, for example, an organohydrogenpolysiloxane containing at least 30 mol % of silicon-bonded hydrogen atoms is added to an addition reaction curable silicone rubber composition whereby the composition is bondable to organic resins. JP-A S63-183843 discloses the integral adhesion of silicone rubber to an olefin resin having grafted thereto a compound having an aliphatic unsaturated group and a silicon-bonded hydrolyzable group. JP-A H09-165516 describes that a thermoplastic resin having added thereto a compound having an aliphatic unsaturated group and a silicon-bonded hydrogen atom is bonded and integrated to silicone rubber. JP-A H09-165517 discloses an integrally molded article in which a blend of a thermoplastic resin and a thermoplastic oligomer containing aliphatic unsaturated groups is integrally molded with an oil-bleeding silicone rubber.
However, heat-curable silicone rubber compositions of the addition reaction type fail to achieve through a brief molding step a sufficient bond to general thermoplastic resins including ABS, PC, PE, PP, PBT, PET, acrylic resins, PA, aromatic PA, PPO, PPS, and urethane resins. To acquire a bonding ability, the resins must be modified as suggested in the above patent documents. Modification of resins requires additional steps and an extra cost, and the modified resins sometimes become vulnerable to quality alteration.
Materials known to overcome these problems include self-adhesive, addition-crosslinking silicone rubber compositions as disclosed in JP-A H06-172738, JP-A 2001-200162 and JP-A 2008-537967. Specifically, an organohydrogenpolysiloxane having a SiH bond and aromatic structure in a molecule is added to the silicone rubber compositions as adhesion improver. The resulting compositions can be briefly molded and bonded to organic resins or metals.
Even when such adhesion improvers are added, adhesion or bondability is reduced depending on a crosslinking system of silicone rubber. Particularly when the rubber hardness exceeds 60 on JIS A scale, adhesion is substantially reduced and sometimes completely lost.